Iraqi Forces Battle Shiite Militias

Upsurge in Violence
Could Pose Threat
To Security Gains

BAGHDAD -- In a worrying resurgence of violence here, Shiite militias and Iraqi security forces clashed Tuesday in Baghdad and in Iraq's southern oil hub, Basra, while the capital's heavily fortified International Zone endured another pounding of rocket attacks.

While the upsurge could be temporary -- coinciding with efforts by Iraqi-government officials to crack down on what they call renegade militias -- it could also represent a new threat to fragile security gains made recently by U.S. forces. If the increase in...